1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a forward link power control device and method for a mobile communication system supporting transmission diversity.
2. Description of the Related Art
For power control in a mobile communication system, a receiving party measures strength of a received signal which varies according to a condition of a radio link. The receiving party then sends the result from the measurement as a power control command to a transmitting party. The transmitting party then increases or decreases power of a transmission signal according to the power control command from the receiving party.
Specifically, to control transmission power of a base station in such a mobile communication system, a mobile station measures a signal-to-interference ratio (SIR), compares it with a threshold and provides the base station with information about the comparison result through a power control bit (PCB) on a reverse channel. Upon receipt of the power control bit, the base station controls power of a transmission signal according to a value of the received power control bit. That is, for forward link power control in a mobile communication system, the mobile station measures power of a signal transmitted from the base station and provides the base station with the result from the measurement, and the base station then controls power of a transmission signal according to the power control information received from the mobile station. This is called a closed loop power control method.
With reference to FIG. 1, there is illustrated a forward link power control procedure in a mobile communication system. Referring to FIG. 1, when a base station transmits a signal 111, a mobile station receives the signal with a propagation delay 113. Upon receipt of the signal, the mobile station measures strength of the received signal and thereafter, sends a power control bit according to the measured result 115. The base station then receives the power control bit with a propagation delay 117 and controls power of a transmission signal according to a received power control bit 119. As illustrated in FIG. 1, power control is performed in a unit of power control group (PCG), and actual power control is performed with a delay of at least one power control group.
As described above, in a mobile communication station, power control is performed in a unit of power control group. The base station sends a signal, and the mobile station then measures an SIR of the signal received from the base station, compares it with a threshold and sends the base station a power control bit generated according to the comparison result. The base station then analyzes the power control bit and controls transmission power for a next power control group according to the analysis. This power control method can be applied to a case where a traffic channel is continuously transmitted via a single antenna.
However, a mobile communication system supporting the transmission diversity requires a new power control method for controlling power of a traffic channel which is switched between two or more antennas. Accordingly, a need exists for a forward link power control device and method for a mobile communication system supporting transmission diversity where transmission signals are outputted via two or more antennas.